Such a process is known from EP 0740002 B1. In this patent publication a process for making a fishing line from yarns of filamentous materials is described, wherein a line made from braided, twisted, or twisted and plied yarns of gel spun polyethylene filaments is exposed to a temperature within the melting point range of said polyethylene for a time sufficient to at least partially fuse adjacent filaments while stretching said line at a stretching ratio within the range from 1.01 to 2.5. Applying such stretch ratio to the precursor during the heat exposure is needed in order to keep the filaments under elongational tension, so as to prevent decrease of the strength of the product as a result of thermal molecular relaxation processes. The yarns applied in this process are high-strength continuous multi-filament yarns, more specifically such yarns made by so-called gel spinning of ultra-high molar mass polyethylene (UHMWPE), for example yarns commercially available under the trademarks Spectra® or Dyneema®. The monofilament-like products thus obtained in EP 0740002 B1 are stated to show less fraying and to have lower surface friction than corresponding braided or twisted lines, while still showing favourable high strength.
In WO 2004/033774 A1 a similar fusion process is applied to a precursor containing a spun yarn made from UHMWPE staple fibres as strand.
Fishing lines are generally monofilaments made from synthetic polymers, having a round, firm structure that allows convenient handling for bait casting, spinning, and spin casting. Such monofilament lines generally have a stiff nature and smooth surface, which combine to reduce drag during the cast and enable longer casts while providing better release from fishing reels. Braided lines containing a multitude of filaments are less suited for fishing lines, because they have a tendency to fray at the end of the line, may entrap water, present an outer surface that is vulnerable to snags and entanglement, and have an opaque appearance that is too visible below water.
The process known from EP 0740002 B1 allows making monofilament-like fishing lines from braided or twisted lines made from polyethylene multi-filaments yarns, which lines have specific advantages over braided lines. The performance of such fused lines also compares favourable with that of a conventional monofilament made from e.g. polyamide by melt extrusion in view of is higher tensile strength (or tenacity) and stiffness. Such thermally fused lines further have an advantage over monofilament-like products made by bonding together multiple filaments with a bonding agent, for example by a melt impregnating step with a thermoplastic polymer like LDPE as described in U.S. Pat. No. 5,601,775, in that they generally show higher tenacity; the strength of the constituting fibres is not ‘diluted’ by the presence of a polymeric bonding agent.
A disadvantage of such fused filamentous lines is their tendency to show pilling: as a result of abrasion of the line, e.g. by moving along guiding members during casting and fishing, surface fused filaments may delaminate, and freed filamentous material rearranges itself on the line into small pills. It is clear that a line showing such pilling will perform less well in casting etc. Therefore, it is desirable to have a monofilament-like product made from a precursor containing fibres made from UHMWPE that combines high tensile properties and knot strength with improved resistance to abrasion, especially showing little pilling.
It is therefore an object of the present invention to provide a process for making a monofilament-like product that does not, or at least to a reduced extent, show said disadvantage.
This object is achieved according to the invention with a process for making a monofilament-like product from a precursor containing at least one strand of fibres made from ultra-high molar mass polyethylene, comprising a) exposing the precursor to a temperature within the melting point range of the polyethylene for a time sufficient to at least partly fuse adjacent fibres and b) simultaneously stretching the precursor, wherein the precursor is mechanically compressed during fusing.
With the process according to the invention a high-strength monofilament-like product can be made from UHMWPE fibres, which product has a smoother surface appearance, and improved abrasion resistance, for example a reduced tendency to pilling during use as fishing line, than known similar products; which makes it very suitable for use as fishing line and the like. A further advantage of the process according to the invention is that very thin monofilament-like products can be made.
The monofilament-like product obtained by the process according to the invention has a pleasant touch or feel and can be easily handled and knotted, and shows very high knot strength and knot strength efficiency. With the process according to the invention it is also possible to make a line with monofilament-like surface appearance, but with flexibility more like a multifilament yarn construction. Such product typically has a sheath-core structure; that is it has a non-porous sheath of fused filaments and a core of mainly filamentous character. A further advantage of the process according to the invention is that it can be applied with high efficiency to twisted and/or air-entangled multifilament yarns, to braided multifilament precursors, as well as to precursors based on short staple fibres; and that it is possible to control formation of said sheath-core structure.